1. Field of the Invention
The present invention relates to electronic devices, and in particular, to clocks for the electronic devices.
2. Description of Related Art
A conventional jitter measurement circuit 10 is shown in FIG. 1 for measuring clock jitter in an integrated circuit (IC) chip. Clock jitter refers to the temporal variation of a clock period at a given point on the chip, i.e., the clock period may reduce or expand on a cycle-by-cycle basis. The circuit 10 has a delay-locked loop (DLL) 12, which receives a clock signal M1 (or its inverted form M2 via a multiplexer 14). The DLL 12 includes a voltage-controlled delay line (VCDL) 16 having 8 adjustable, cascaded DLL delay elements 18 (two inverting half delay circuits, for example) for delaying the clock signal in increments of the adjustable delay DLY so as to generate a reference signal; a phase detector array 20 using one of a plurality of phase detectors for comparing the clock and generated reference signals to generate a phase difference indicative of the clock jitter; and a feedback path 22 responsive to the phase difference to adjust the delay through the VCDL 16 such that the rising edges of the two signals at inputs of a selected phase detector are aligned and locked. The feedback path 22 typically includes the selected phase detector of the array 20, a digital low-pass filter 24, an up/down counter 26 and a bias generator 28. A jitter recorder 30 displays the measured phase jitter. For calibration purposes, a switch 32 and a calibration reference signal generator 34 (to provide a calibration reference signal) are included.
Referring to FIG. 2, the conventional phase detector array 20 is shown. The array 20 includes 16 phase detectors 40, in the form of flip-flops, which are designated by numbers 0–15 with only phase detectors 40 with numbers 5–9 being illustrated. A first line 41, which has 8 array delay elements 42 (only delay elements 42A and 42B are shown), receives the generated/calibration reference signal (hereinafter, simply reference signal) and progressively delays the reference signal before it passes to the phase detectors with numbers 8–15. A second line 43, which has a 7 of array delay elements 44 (only elements 44A and 44B are shown), receives the clock signal and progressively delays the clock signal before it passes to the phase detectors with numbers 6–0. At phase detector identified by #7, the clock and reference signals are not delayed with respect to each other by an array delay element. By default the output of phase detector #7 is selected as the feedback signal to the low-pass filter 24 of FIG. 1; hence, the edges of the clock and reference signals are aligned on row #3. Each of the phase detectors 40 compares the reference signal to the clock signal, with varying delays between the two dictated by the array delay elements 42 and 44. Outputs of adjacent phase detectors 40 are compared by XOR logic gates 46 (with the exception of the XNOR logic gate 48) to generate phase bin output of 1 or 0, which is fed to phase bins (not shown) in the jitter recorder 30 of FIG. 1. The outputs of the logic gates 46 and 48 indicate which phase bin has received a hit.
Five illustrative Scenarios −2 through +2 are shown in five columns of FIG. 2, with Scenario 0 at the center column having no phase jitter and each Scenario on either side of the center column representing a progressively increasing amount of phase jitter in opposite directions. The rising edges of the clock and reference signals are shown with by a matrix of wiggly lines for each of the 5 Scenarios (columns) and each of the phase detectors identified by numbers 5–9 (Rows). For a given Scenario and Row, a dashed edge indicates where the rising edge would be if there was no clock jitter and a solid edge indicates where the rising edge would be with the amount of clock jitter in that Scenario. Hence, in the columns of Scenarios −1 and −2, either the clock edges (Rows 1 and 2) or reference edges (Rows 4 and 5) are progressively delayed, and in the columns of Scenarios +1 and +2 either the clock edges (Rows 1 and 2) or the reference edges (Rows 4 and 5) are progressively advanced. Outputs of 1 or 0 from the phase detectors 40 are specified under the clock edges and the outputs of 1 or 0 of the logic gates 46 and 48 are interposed in the Rows laterally to the logic gates 46 and 48. Hits received by the phase bins are shown by 1's in boxes. For example, in Scenario 0 all the outputs from the phase detectors identified by numbers 0–15 are 0 and there is a single phase bin hit in Row three, i.e., 1 output from the XNOR logic gate 48 having 0 inputs from the phase detectors identified by numbers 6 and 7. All of the other bins do not have a hit. Generally, different patterns of hits indicate or detect a range of scenarios. The scenarios illustrated in FIG. 2 are the boundary cases of each range. For example, any jitter scenario between Scenario 0 and Scenario 1 would result in a pattern whereby the XNOR logic gate 48 is the only one hit.
The pattern of bin hits is a way to detect a certain kind of Scenario, but these Scenarios are not limited to detecting amounts of clock jitter. In the prior art, the same phase detector array 20 is used for calibration. Although simulations are used to estimate the delays of the array delay elements 42 and 44, calibration is required to measure their actual values. During calibration of the phase detector array 20, the feedback path 22 is opened by the switch 32 and a calibration reference signal from the external calibration reference signal generator 34 is selected by a multiplexer 36 to be used instead of the generated reference signal from the VCDL 16. How this calibration procedure works may be illustrated by Scenario −2, Row 2 showing the output for phase detector #8 of FIG. 2 wherein the rising edges of the delayed clock signal and the reference signal are substantially aligned due to the clock jitter of Scenario (−2), unlike the relative edge positions shown in the other columns of Row 2. During calibration, an operator, in attempting to determine the delay of the array delay element 42 between phase detectors #7 and #8, varies a phase offset to the calibration reference signal from the generator 34, in a manner illustrated in the various positions of the Row #2, to accomplish the same alignment shown Scenario −2, Row #2. Upon alignment, the bin between phase detectors #8 and #9 receives a hit, which is noted by the operator. Hence, the misalignment of the clock and calibration reference signals is substantially canceled by the delay of the array delay element 42 between phase detectors #7 and #8. Hence, the known misalignment offset introduced by the operator (instead of by phase jitter) determines the delay of this array delay element 42. This procedure is repeated for each of the array delay elements 42 and 44.
The external calibration reference signal generator 34 is expensive and the calibration procedure has limited accuracy. Also, bandwidth limitations of external routings, i.e., packaging for the chip and other interconnects, degrades the external calibration reference signal.